The present invention relates to a device for and a method of pump light supply for laser active and/or amplifying fibers.
For excitation of a laser light emission of a so-called fiber laser, two different possibilities are known. A so-called pump, or in other words an energy excitation can be provided either by a longitudinal or a transverse supply of excitation lights. For excitation of fiber lasers and fiber amplifiers exclusively longitudinal pumping is known. A basis for this is provided, in addition to other aspects, in the length-diameter ratio of the fibers. Since a fiber core typically is available over a diameter of at least 10 .mu.m, therefore during a transverse pumping only an exceptionally low fraction of the pumping efficiency can be absorbed. For example a laser core with a diameter of 7 .mu.m, (single-mode laser or amplifier for a utilization with 1 .mu.m wave length) absorbs during a single-time passage only substantially between 0.1% and 1% of the pump power, and this fraction depends on an ion doping concentration of the fiber laser or amplifier.
Also, a method of longitudinal coupling of the pump light in the laser core, in particular for high power fiber laser and amplifier, is known with output conductors of several watts. Since the radiation quality of the pump laser diodes for a direct coupling of the pump power in the region doped with a laserion (so-called single-mode region) of the fiber is not sufficient, high light powers must be realized only through fibers with multiple cores.
Patent document WO 95 10868 shows a method for a repeated supply of pump light in so-called double core fibers. The fibers are composed of multiple cores which are arranged coaxially with one another. In one pump core with a typical diameter of 100 .mu.m-400 .mu.m, the power of the laser diodes is coupled. The diameter of the pump core has a plurality of diameters of the single mode laser cores, typically between 5 .mu.m and 10 .mu.m. Advantageous for the double core fibers when compared with conventional pump technique, is the possibility to realize fiber lasers with only one occurring spacial mode of the irradiated laser light, as well as a very high pump light aperture and pump light coupling surface.
The disadvantage of the known double core method is the expensive and complicated and therefore cost intense manufacture of the multiple core fibers. Since the fibers additionally must be optimized, for example with respect to polarization preservation and a radiation strength, there are limitations with respect to the materials to be used for the fibers. In the pump core the occurring radiation modes which have no spacial overlapping with the laser core must be suppressed. This can be performed by spacial winding technique or pump core geometries. Moreover, the known double core techniques for a beam formation of the laser diodes are technically expensive and have the risk of insufficient reliability, which in many applications, for example for space applications can be problematic. In addition, a simultaneous coupling of signal and pump sources is required in a double core fiber for amplifier applications of a wave-length-selective coupling, whereby typically power losses occur up to 50%.